The present invention relates to a pressure-sensitive conductive ink composition used for pressure-sensitive sensors and the like which measure, for example, pressure and/or its distribution.
A conventional pressure-sensitive conductive ink composition, mixed with an ink binder in which a thermoplastic resin is dissolved by heating in a high boiling solvent, has been used for pressure-sensitive sensors and the like.
Referring to FIGS. 2 and 3, a pressure-sensitive sensor 1 includes, at one side, a sheet substrate 4 on which line electrodes 2 are printed at predetermined intervals. A pressure-sensitive ink 3 is applied to surround line electrodes 2. At the other side, a sheet substrate 7 has row electrodes 5 printed perpendicular to line electrodes 2. A pressure-sensitive ink 6 is applied to surround row electrodes 5. The surfaces of the respective pressure-sensitive conductive ink layers are opposed to each other.
In the conventional pressure-sensitive conductive ink compositions, however, there is the problem with maintaining the desired pressure sensitive characteristics in a high-temperature atmosphere.
It is an object of the present invention to provide a pressure-sensitive conductive ink composition which overcomes the foregoing problems.
More particularly, it is an object of the present invention to provide a pressure-sensitive conductive ink composition capable of maintaining pressure-sensitive characteristics in a high temperature atmosphere.
Briefly stated, the present invention provides a pressure-sensitive conductive ink composition including a thermoplastic resin and a conductive filler. The thermoplastic resin is added with a curing agent capable of a partial crosslinkable reaction. The resulting composition maintains its pressure-sensitive characteristics, even at elevated temperatures.
To achieve the objects of the present invention, the following technical measures have been undertaken by the inventors.
(1) The pressure-sensitive conductive ink composition of the present invention has a thermoplastic resin and a conductive filler. The thermoplastic resin is added with a curing agent capable of a partial crosslinking reaction.
In the present invention, the thermoplastic resin undergoes a partial crosslinking reaction by the curing agent to be thermoset, thereby improving the glass transition point.
(2) The thermoplastic resin may have polyhydroxy ether. This provides an ether linkage instead of an ester linkage, as ester linkages are inferior in hydrolyzability. This results in an improved humidity resistance.
(3) The pressure-sensitive conductive ink composition of the present invention may also have a non-conductive filler.
(4) The polyhydroxy ether may have a structure of the following general formula (I), preferably synthesized from bisphenol A and epichlorohydrin. 
wherein n is from about 1 to about 500, preferably from about 50 to about 250, even more preferably from about 80 to about 150.
This polyhydroxy ether structure gives the composition an advantageous improvement in heat resistance.
(5) The polyhydroxy ether may alternatively have a structure of the following general formula (II), preferably synthesized from tetrabromophenol A and epichlorohydrin. 
wherein n is from about 1 to about 500, preferably from about 50 to about 250, even more preferably from about 80 to about 150.
This polyhydroxy ether structure advantageously improves the heat resistance while maintaining the flexibility and imparting a flame retardancy.
(6) The curing agent may be one or more isocyanates selected from the group consisting of TDI (tolylene diisocyanate), IPDI (isophorone diisocyanate), HDI (hexamethylene diisocyanate) and XDI (xylylene diisocyanate). These curing agents advantageously improve heat resistance.
(7) The curing agent may be one or more polyisocyanates selected from the group consisting of burette-type, adduct-type and isocyanurate-type isocyanates of TDI (tolylene diisocyanate), IPDI (isophorone diisocyanate), HDI (hexamethylene diisocyanate) and XDI (xylylene diisocyanate). These curing agents improve durability while providing easy handling (in toxicity) at the time of manufacturing.
(8) The curing agent may make use of one or more active hydrogen compounds selected from the group consisting of methyl ethyl ketone, xcex5-caprolactone, ethyl acetoacetate and phenol as a blocking agent. These blocking agents advantageously give a longer shelf life.
(9) The composition may be blended such that the ratio of NCO in the isocyanate curing agent to OH in the polyhydroxy ether (NCO:OH) is preferably about 0.2 to 2.0. This ratio enables the desired rate of the crosslinking reaction.
(10) The conductive filler may be conductive carbon black with a developed structure having an amount of DBP oil absorption of about 200 to 480 ml/100 g. This conductive filler enables the composition to obtain stable conductivity.
(11) The non-conductive filler may be superfine silicon dioxide having a primary particle diameter of about 7 to 40 nm. This enhances the dispersibility of the carbon black as well as the printability of the composition.
(12) The pressure-sensitive sensor of the present invention employs the pressure-sensitive conductive ink composition as described above.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.